A. Field of the Invention
This invention relates to the can manufacturing art, and more particularly to a novel construction and arrangement of the tooling that is used to form the seam joining a can end to a can body.
B. Description of Related Art
It is well known to draw and iron a sheet metal blank to make a thin-walled can body for packaging beverages, such as beer, fruit juice or carbonated beverages. In a typical manufacturing method for making a drawn and ironed can body, a circular disk or blank is cut from a sheet of light gauge metal (such as aluminum). The blank is then drawn into a shallow cup using a cup forming punch and die equipment. The cup is then transferred to a body maker or can forming station. The body maker draws and irons the sidewalls of the cup to approximately the desired height and forms dome or other features on the bottom of the can. After formation of the can by the body maker, the top edge of the can is trimmed. The can is transferred to a necking station, where neck and flange features are formed on the upper region of the can. The flange is used as an attachment feature for permitting the lid for the can, known as an “end” in the art, to be secured to the can.
The end is the subject of a different manufacturing process and involves specially developed machines and systems to manufacture such ends in mass quantities. After the ends are formed, they are sent to a curling station where a peripheral curl is provided to the end. As will be discussed below, the peripheral curl is used in a seaming operation to join the can end to the can body. After curling, the ends are sent in stick form to a compound liner station. A water-based compound sealer is applied to the ends in the compound liner station. From there the ends are fed to an inspection station and to a dryer station where the compound is subjected to heated forced air to dry the compound. If a solvent-based compound is used, then no drier is needed. The ends are then placed in stick form, bagged, and then loaded on pallets for shipping.
The preservation of the contents of the can requires the formation of hermetic seal between the end and the can body. The can must also resist internal and external pressures. These internal pressures include the pressure due to carbon dioxide gas contained in carbonated beverages, beer and the like. These pressures must be contained by the seam or joint attaching the can end to the can body. Generally speaking, most bottlers require that the seam must withstand an internal pressure of 90 PSI (pounds per square inch), although some require more. Furthermore, the seam must prevent the leakage of gas from the container. Today, the hermetic seal between the can end and the can body is typically formed as a result of a process known in the art as double-seaming. In this process, the can end peripheral curl and can body flange are held together, interlocked, curled, and roll-pressed to form a hermetic seal.
The double seaming operation uses two successive seaming operations, a first operation using a first seaming roll and a second operation using a second seaming roll. The process is illustrated in FIGS. 1A-1D. In the first operation, the end 10 and can body 12 are clamped together between a lifter mechanism (not shown) and a seaming chuck 14. The seaming chuck has a configuration profiled to fit within the can end, generally within the countersunk portion 16 of the end 10. The seaming chuck 14 includes an anvil portion 18 and an upper peripheral wall 20 that supports the flange 22 of the can body as shown in FIG. 1C. The can body 12 and end 10 rotate at high speed about the longitudinal axis of the can body while a first seaming roll 24 is moved into contact and brought to bear with a steady pressure against the peripheral curl 32 of the can end 10, as shown in FIG. 1C. The first-operation seaming roll 24 is mounted on a bearing which, when the roll 24 is pressed against the peripheral curl, allows the roll 24 to freely roll in a counter-rotational direction as the can body and can end are rotated about the longitudinal axis of the can body.
The upper face 26 of the first-operation seaming roll 24 just clears the top lip 30 of the seaming chuck 14. As the roll 24 presses against the end's curl 32, the flange 22 of the can bends over to form a body hook 34. The curl 32 of the end tucks underneath and behind the body hook 34 to form the cover hook 36.
The function of the second-operation roll 40 is to complete the seam formation. It does so by compressing the cover hook 36 and body hook 34 tightly against the anvil of the chuck 14, so that the two interlock tightly, as shown in FIG. 1D. The gaps between the two are filled with the sealing compound originally placed inside the curl of the end 10. The result is a strong, leak-proof seal between the can body and the end.
Can end manufacturers are continuously striving to reduce the amount of metal used to form the can ends. Currently, such efforts include can ends made of thinner gauge metal, and designing the ends with larger inclination angles between the chuck wall of the can end and the longitudinal axis. One such can end is described in Brifcani, et al., U.S. Pat. No. 6,065,634, an example of which is shown in FIG. 2A. In the '634 patent, the chuck wall 42 is said to be inclined at an angle of between 40 and 60 degrees. When the end of the '634 patent is double seamed to a can body using existing seaming equipment, this chuck wall is constrained to be inclined at a 4 degree angle relative to the longitudinal axis, plus some small additional angle due to springback of the chuck wall 42 after double seaming. This is because conventional seaming chucks typically have an upper anvil wall angle of 4 degrees to vertical, as shown in FIG. 1B. The present inventors have observed that for cans ends with relatively large chuck wall angles (greater than say 25 degrees), this large an angle for the chuck wall tends to produce a seam gap and a resulting force F between the can end and the can body that has a tendency to un-do, or weaken, the double seam, as indicated by the arrow in FIG. 2B. Such a gap and associated weakening of the seam also has a tendency to reduce the buckle strength of the can end. In particular, the buckle strength (i.e., the limit at which the double seam fails) can fall below the minimum level that is acceptable.
FIG. 3 is a cross-sectional view of a prior art second seaming roll 40 showing an arcuate seaming profile. The seaming roll has a seaming surface 52 that contacts the curl of the end as shown in FIG. 1D, and an upper face 26 that clears the top surface 30 of the chuck, and a radius R1 that connects the seaming surface 52 at its upper edge 54 to the face 26. The lower edge 56 of the seaming surface 52 is connected to a radius R2 that supports the lower curl edge 58 of the double seam, as shown in FIG. 1D. In accordance with the definitions of the incline or slant angle of the seaming surface described below, the seaming surface 52 of the standard prior art roll is inclined slightly, at a 2 degree angle. This angle is determined by constructing a chord 60 that connects the upper and lower edges 54 and 56, and measuring the angle β between this chord and a line 62 parallel to the longitudinal axis of the can body. Line 62 is also parallel to the axis of the seaming roll 40 and parallel to the axis of revolution of the can body and end during the double seaming. R1 and R2 can be anything as long as they are tangent to the seaming surface at the upper and lower edges thereof (i.e., so that a continuous surface is formed for R1, R2 and the seaming surface 52). Chord 60 is only relevant from the standpoint of providing an analytical technique for determining the vertical inclination of the seaming roll, in accordance with this invention.
The present invention provides improved seaming apparatus and methods that provide improved buckle strength in the double seam. While not necessarily so limited, the invention is particularly advantageous in the seaming can ends in which the chuck wall of the can end is inclined relative to the longitudinal axis at relatively large angles prior to seaming, such as angles of between 20 and 60 degrees.